Oct 13

Understanding Exposure – part 1 – Introduction

Under-exposed, properly exposed, and over-exposed images

This is the first in a series of articles about how exposure works in digital cameras. If you have a digital camera that can be put into the three “Creative Modes” – Shutter Priority, Aperture Priority, and Manual Mode – then this article will give you the basic understanding of exposure to make it work to your best advantage. You don’t need a DSLR for this – the information is equally valid for compact cameras. Point-and-shoot owners, or iPhone users should probably concentrate on composition as the exposure in these cameras doesn’t usually allow you to take control of your exposure.

You’ve got a compact camera or DSLR and it works just fine in Programmable Mode, so what are all these other modes good for? Shutter Priority, Aperture Priority and (shudder) Manual Mode – what’s the point?  The truth is that you can take a perfectly good photograph using either the Auto or Programmable Modes, but unless you understand what these other “creative” modes bring to the table, you may be missing out on a lot of the flexibility your camera offers and the ability to create photos that will set you apart from the crowd and allow you to create a photo instead of just taking a photo.

Taking a photo is easy. You just point and click. Today’s cameras are really excellent at setting a usable shutter speed. They may turn on the flash when you need it, and usually grab a good image of your subject. For many people, that’s enough.

Creating a photo means that you are in control of the image and are making conscious choices about how that image will appear. In some cases, that may mean putting your camera into Programmable Mode! But that should be a choice, not a lifestyle; and in order to make an informed choice you will need to know how the basic settings of your camera conspire to give you a good exposure.

Exposure Basics

There are three components that work together to deliver a good exposure: Shutter Speed, Aperture, and ISO. Of these, only shutter speed is anywhere near obvious in its meaning and intent, but knowledge of all three is key to understanding why a photo appears the way it does. There is no such thing as THE perfect exposure. There are actually many combinations of shutter speed, aperture, and ISO that will yield perfect exposures. When the camera is in Auto or Programmable mode, it will balance two or even all three of these to yield one perfect combination – if it can. Usually the fully automatic modes will try to pick a mid-range aperture – around f/8 – but won’t allow the shutter speed to drop below 1/60 sec, if possible. The result is probably a good photo, but all your photos tend to look similar in style because the fully automatic modes are designed to produce a consistent exposure following some simple rules. But many other exposure combinations are possible, and they all look slightly different.

Consider the scene below. Each image is a perfect exposure of the same scene, but they look very different (click to enlarge).

Same scene, three different apertures. Notice how the background changes.

The scene on the right is typically what you would get if you took the picture with a point-and-shoot camera. Most everything is in focus and nothing is isolated visually from anything else. This can be a problem in a photograph because, unlike our normal stereoscopic sight, this photograph lacks depth – you can’t easily distinguish what is close from what is further away without factoring in how large you know things to be, or if one thing obscures another. What looks good to you in person may get lost in the background because the camera can’t distinguish the subject. The scene in the middle is what a camera might take if it were in Programmable or Auto mode. These modes strive to take a one-size-fits-all approach that might blur the background a little, but has a good depth of field. This helps to identify what the photographer was trying to take a picture of, but may not be enough of a visual cue for the viewer. In the photo on the left, the focus is clearly on the flower, so there is no doubt what the photographer wanted to draw your attention towards. Each of these exposures are perfect, and each have their place. Your job is to learn how to deliberately create each type of photo and then decide for yourself which is appropriate for the image you want to capture.

A perfect exposure can be likened – albeit poorly – to a bucket that is filled with water using a hose. The goal is to fill the bucket – and that represents a perfect exposure. Light is analogous to water in this case. The amount of time it takes to fill the bucket is the shutter speed. The size of the hose is like the aperture: a fire hose will not take as long to fill the bucket than a garden hose will, because the wider fire hose will allow much more water to flow than a narrow garden hose. The size of the bucket is determined by the ISO. The higher the ISO number, the smaller the bucket is – in other words, it doesn’t take as much water to fill it.

Using this analogy, you can imagine that if it takes 30 seconds to fill a bucket with water from a garden hose, it might only take a second or two to fill it from a fire hose. If the bucket were smaller, then it could be filled faster without using a larger hose, and so forth.

In future posts I will examine each of these exposure components in turn and I will show what each of them does and why you might want to take control of them to help make a more compelling photograph.

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